1. Field of the Invention
The present invention relates to an image data processing system, and more particularly, to a method, apparatus and computer program product for processing image data acquired by an image capturing device, such as a digital camera, etc.
2. Discussion of the Background
Japanese Patent Laid-Open Publication No. 4-168879 discloses an image forming apparatus in which the printing operation is performed based on a video signal. In the image forming apparatus, video signals are sampled and an image is determined to be overexposed when a number of samples, which have a value equal to a specified threshold value TH or greater, is equal to a predetermined value NH or greater. A tone conversion of the video signal is then performed using a suitable tone conversion curve. It is determined that the image is photographed with flash light when the number of samples, which have a value equal to a specified threshold value TL (<TH) or lower, is equal to a predetermined value NL or greater. The tone conversion of the video signal is performed using a suitable tone conversion curve.
Further, Japanese Patent Laid-Open Publication No. 63-184473 discloses an image forming apparatus in which the luminance histogram of image data is used to switch the tone correction table based on the luminance condition of the image data. Two histograms are generated i.e., one for a high luminance region and the other for a low luminance region, to reduce the memory capacity required for generating the histogram. An image capturing device, such as a digital camera, generally includes an automatic exposure control mechanism to obtain an optimum exposure. Generally, three systems are employed as the automatic exposure control mechanism, Namely, an average, center-weighted, and spot metering systems. In the average metering system, the amount of light is measured by dividing the image screen into multiple regions. The exposure is controlled based on a weighted average value of the amount of light of the divided regions. In the center-weighted metering system, the amount of light in the center region of the image screen is mainly measured. In the spot metering system, the exposure is controlled by measuring the amount of light in a local spot of the image screen.
A proper exposure adjustment may not be easily made even in an image capturing device having the above-described automatic exposure control mechanism when an image is photographed under backlight or partly under backlight conditions.
For example, in a true backlight condition in which the sun is located just behind a subject, the subject is darkened (i.e., underexposed) in the average or the center-weighted metering systems because the luminance difference between the background and the subject is substantial. The background part, e.g., the sky is whitened (i.e., overexposed). In the spot metering system, the exposure is controlled such that the subject is not underexposed in the true backlight condition. However, the background e.g., the sky is tends to be overexposed. Further, even in the spot metering system, the correct exposure for the subject is not always obtained under various photographing conditions because the position and the size of the photometry frame in the image screen are fixed.
A halation phenomenon in which light from the light source enters into the photographic lens also often occurs. The halation tends to occur frequently when a landscape is photographed in the morning or in the evening when the sun is low in the sky. In the halation condition, the subject itself is correctly exposed, although a part of the image is bleached-out, because the light source is not located immediately behind the subject.